By now you have associated various chords with a given "Key". We just covered the Key of "E" and the associated E Major, A Major and B7 chords. The illustration above details additional chord members within a given Key. For example, the Key of "C" we have added D minor (II) and A minor (VI). These chords are built on the 2nd and 6th degree of the "C" Major scale.
These additional chords often appear within the parent key. That is, melodies in the Key of "C" use an A minor and/or a D minor chord. Begin to associate these extra chords with the melodies that we have and will cover.